A combination of a banana (an Asian who is yellow on the outside and white on the inside) and a slut. Banana slugs are Asian girls who try too hard to prove that they aren't a school-girl stereotype, thus confusing being white with being promiscuous. In addition to their hooched up outfits, they often have a combination of badly dyed yellow-brown hair, fake eyelashes, dark tans, and pretend to speak no language but English.

"Did you hook-up with that hot Asian from the bar?"
"Yeah, it was easy, turned out she was a banana slug who wants to prove herself and punish her daddy at the same time."

A wet, flaccid penis, usually after it's been freshly removed from a vagina, mouth, or rectum; thus resembling a slimy little slug. Can be the cause or consequence of ending sex.
(May also be used in verb form - see below)

Example 1:
To everyone's horror, Chris walked out of the bedroom naked with a satisfied expression and an exposed banana slug.

Example 2:
"I wouldn't have stopped pounding her yet, but my drunk was wearing off and I was pretty much banana slugging it by that point."